Pogue, Jr. — Geology and Structure of Volcanic Pocks. 219 



North Carolina was recognized. In that year, George II. 

 Williams,* in a paper on the distribution of ancient volcanic 

 rocks along the eastern border of North America, announced 

 their identification in Chatham County and near Chapel Hill, 

 and suggested their probable wider distribution. Since then, 

 Becker.f Nitze and Hannah Weed and Watson, § and Graton|| 

 have described in a general way ; and Laney,!" and after him 

 the writer, have considered in detail areas of these rocks and 

 established their wide occurrence. 



General Geology, 



Outline. — The portion of the Piedmont Plateau herein 

 described exposes the beveled folds of a great volcano-sedi- 

 mentary formation. A traverse across the district from north- 

 west to southeast passes over the eroded edges of once 

 horizontal beds, which show upon the surface as elongated 

 belts and lenses. Their character indicates an origin during 

 a period of great volcanic activity. 



Wide bands of a sedimentary, slate-like rock, composed of 

 varying admixtures of volcanic ash and land waste, have the 

 greatest areal extent. Intercalated with these occur strips and 

 lenses of acid and basic volcanic rocks, represented by fine and 

 coarse-grained volcanic ejectamenta and old lava flows. The 

 acid rocks include fine tuffs, coarse tuffs, and breccias, chiefly 

 of a rhyolitic and dacitic character, together with flows of 

 rhyolite and dacite. The basic series embrace fine tuffs, coarse 

 tuffs, and breccias of an andesitic nature, and flows of an 

 andesitic and trachy-andesitic stamp. Gabbro and diabase 

 dikes cut the other formations. 



The region has suffered a period of severe dynamic meta- 

 morphism, consequent upon a great- compressive force which 

 squeezed the beds into enormous folds ; followed by a time of 

 chemical alteration and mineralization ; which in turn was 

 succeeded by a long period of erosion and weathering. The 

 rocks have suffered to a variable degree from all these factors. 



* Williams. George H., Distribution of Ancient Volcanic Rocks along the 

 Eastern Border of North America. Jour. Geol.. v. ii, 1-31, 1894. 



f Becker, G. F., Gold Fields of the Southern Appalachians. (In U. S. 

 Geol. Survey, 16th Ann. Rept, pt. 3, 1895.) 



% Nitze, H. B. C, and Hanna, G. B., Gold Deposits of North Carolina. 

 N. C. Geol. Survey, Bull. 3, 1896. 



jjWeed, W. H, and Watson, T. L., The Virginia Copper Deposits. Ec. 

 Geol., v. i, pp. 309-330, 1906. 



|| Graton, L. C, Reconnaissance of some Gold and Tin Deposits of the 

 Southern Appalachians. U. S. Geol. Survey, Bull. 293, 1906. 



s Laney, F. B.. The Gold Hill Mining District of North Carolina. A 

 Thesis, Yale University, 1908. 



Laney, F. B., and Pogue. J. E., Jr., An Outcrop Map of the Virgilina Cop- 

 per District of North Carolina. Scale 1 : 24,000. N. C. Geol. Survey, 1908. 



